I think not I'm the one who has 1200' contacts, and after Microsoft mixed all my Skype, Facebook and MSN contacts' details, for some reason my Calendar has been flooded with - how to say nicely - not too particularly important individuals' birthdays. And even worse, it keeps begging for attention by sending 2-3 notifications about these events.
The biggest problem with the new Windows Modern UI is, that it is really ineffective in a way to let the user change the settings. I'm a Windows user since I was 12, but I constantly has to remind myself that each Metro app must have a settings panel somewhere - from the UI perspective alone it is a complete VOID of options - and it is not self explanatory.
tldr; by opening Calendar and going to the Settings charm menu (right side of the screen), from the top the second menu is "Settings" or "Properties" (my guess only - mysteriously this menu shows up as Hungarian on my machine, however I did *not* install any language packs or anything). There you'll find the Birthday calendar pane, where you can disable this feature.

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